1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the same.
2. Description of Related Art
A self aligned contact (SAC) method has been employed in the pasts to achieve a higher degree of integration in semiconductor devices.
The SAC method is as follows. First, gate electrodes having side wall spacers and offset nitride films (silicon nitride films used for offset) are formed on a substrate. An interlayer insulation film is then formed so as to cover the side wall spacers, offset nitride films, and gate electrodes. After this, contact holes are formed at self-aligning positions utilizing the etching selectivity of the side wall spacers, the offset nitride films, and the interlayer insulation film.
With the miniaturization in design rules today, the exposure mask alignment margin in photolithography is determined by the alignment or mask precision of the apparatus, among other factors. It is therefore considered difficult to achieve a mask alignment margin of less than about 0.07 to 0.08 μm. The SAC method, however, is suited to increasing the integration of a semiconductor device in that the mask alignment margin can be larger.
The method disclosed in publication I (Japanese Laid-Open Patent Application H10-4190) is an example of a conventional SAC method.
FIG. 8 serves to illustrate a conventional SAC process. The SAC process in publication I will now be briefly described through reference to FIGS. 8(A) to 8(D).
As shown in FIG. 8(A), first, a gate insulation film 101 and a plurality of gate electrodes 105 having offset nitride films 103 are formed on a substrate 107. With the technique in publication I, the next step is to form side wall spacers 109 composed of silicon nitride film at the side wall 105a portions of the gate electrodes 105, as shown in FIG. 8(B). An interlayer insulation film 111 composed of a silicon oxide film, for example, is then formed, as shown in FIG. 8(C).
After this, as shown in FIG. 8(D), an etching mask 113 having openings corresponding to contact holes is formed. The interlayer insulation film 111 is then etched via this etching mask 113. As a result, just the interlayer insulation film 111 is selectively etched with respect to the silicon nitride films (the offset nitride films 103 and the side wall spacers 109). At this point there remain portions protected by the side wall spacers 109 and the offset nitride films 103. In this way contact holes 115 are formed at self-aligning positions in the interlayer insulation film 111.
Unfortunately, with the SAC method of publication I, as shown in FIG. 8(D), the offset nitride films 103 and side walls 109 are formed from silicon nitride films. Consequently, a hot carrier phenomenon tends to occur when the semiconductor elements in the semiconductor device are driven, as disclosed in publication II (T. Mizuno et al., “Hot-Carrier Injection Suppression Due to the Nitride-Oxide LDD Spacer Structure,” IEEE Transactions on Electron Devices, Vol. 38, No. 3, p. 584, March 1991) or publication III (F. C. Hsu and H. R. Grinolds, “Structure-Enhanced MOSFET Degradation Due to Hot-Electron Injection,” IEEE Electron Device Letters, Vol. EDL-5, No. 3, p. 71, March 1984).
According to publications II or III, the hot carrier phenomenon tends to occur in the vicinity of the interface between the silicon oxide film and silicon nitride film (the portion circled with a broken line in FIG. 8(D)). The hot carrier phenomenon is particularly prone to occur if the silicon oxide film/silicon nitride film interface is close to the substrate. Therefore, a drawback to the semiconductor device in publication I is its large hot carrier injection.
FIG. 9 serves to illustrate an improved version of prior art. In order to suppress hot carrier injection in the technique of publication I, it is proposed, for example, that a thin oxide film be formed over the entire side wall portion of the gate electrodes 105 prior to the formation of the side wall spacers 109 composed of silicon nitride film. In the example shown in FIG. 9(A), gate electrode side wall oxide films 117a are deposited on the surface of the gate electrodes (polysilicon gate electrodes) 105 and the offset nitride films 103 in the example shown in FIG. 9(B), thin silicon oxide films 117b are formed on the side walls of the gate electrodes 105.
At the present time, however, as miniaturization is increasing due to higher integration, the gap between adjacent gate electrodes 105, for example, is already down to about 0.20 μm. Furthermore, side wall spacers need to be provided to each gate electrode. The slit gap formed between opposing side wall spacers is therefore only about 0.10 μm or less, for example. In this case, as shown in FIG. 9, it is difficult to form the contact holes 115 in self-aligning fashion, as mentioned above. Specifically, the narrow slit gap w tends to cause a phenomenon whereby etching comes to a halt at the interlayer insulation film 111 in this fine slit portion (generally called etch stop). Thus, with the prior art shown in FIG. 9, it is difficult to form the side wall spacers 109 while ensuring a slit gap w that is wide enough to prevent etch stop. Consequently, poor contact tends to occur.